RFC: extern illegal instruction trap and trap RDCYCLE

[Ben Dooks]

On 17/09/2024 14:46, Palmer Dabbelt wrote:
> On Tue, 17 Sep 2024 06:08:50 PDT (-0700), ben.dooks at codethink.co.uk wrote:
>> This is a RFC series to change how the illegal instruction trap
>> is handled and then how to trap RDCYCLE and emulate it with RDTIME
>> instead.
> 
> Only 1/3 made it to lore for me, not sure if it's just stuck somewhere.
> 
>>
>> I did this when we found multiple libraries using RDCYCLE and
>> upgrading multiple runners to newer kernels caused many problems
> 
> OK, so I think we're kind of just stuck with RDCYCLE then -- it was part 
> of the base ISA when we merged the port, and every time it disappears we 
> end up breaking userspace.

Yes, it was an annoyance, we managed to track down all the users and
patch out.

I thought this might be a useful idea for more generic type, so might
look at updating to have a table of instruction masks to call.

> I'm not sure what exactly the right way to do this is: IIRC there's some 
> perf-related hooks for this, but there's also systems that just don't 
> implement the RDCYCLE instruction at all and thus we'll need some sort 
> of emulation for those.

I couldn't get the PMU driver to allow it, not sure if there was an
issue higher up or some other issue?

> So hopefully the other two patches get through the lists at some point, 
> but I think in general this is a reasonable thing to do -- or I guess 
> maybe a completely unreasonable thing to be stuck needing to do, but no 
> way around it ;)

I'll check back later, currently as OSS-EU

>>
>>
>>
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-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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